Moldable metal particles take essentially one of two forms, i.e., encapsulated and unencapsulated. Encapsulated metal particles comprise a metal core encased in a polymeric shell, and are compression molded to form a variety of products. For example, soft magnets are molded from polymer encapsulated ferromagnetic particles such as iron, and certain silicon, aluminum, nickel, cobalt, etc., alloys thereof (hereafter "iron"). Such soft magnets are readily demagnetized with less than about 200 Oersteds coercive force. Polymeric shell materials useful for such soft magnets include thermoplastic polyetherimide, polyamideimide, polyethersulfone, polycarbonate and polyphenylene ether, inter alia. U.S. Patent Ward et al. 5,211,896 discloses one example of such a material wherein the polymeric shell is spray-coated from a solution thereof to form a continuous film on the surfaces of the metal particles. Permanent (i.e., hard) magnets are also known to be compression molded from such ferromagnetic particles as magnetic ferrites, rare-earth metal alloys (e.g., Sm-CO, Fe-Nd-B, etc.), and the like spray-coated to form a polymeric shell. Shain et al. 5,272,008, for example, discloses one such hard magnet-forming material comprising iron-neodymiumboron particles encapsulated in a spray-coated composite polymeric shell comprising an epoxy underlayer overcoated with a polystyrene outer layer. Other metals and polymer shells have been proposed for various applications. Unencapsulated metal particles, on the other hand, have no such polymer shell, and are used primarily in the manufacture of sintered products.
In Ward et al., and Shain et al. supra, the shell-forming polymers are completely dissolved in an appropriate solvent, and a fluidized stream of the metal particles spray-coated with the solution, using the co-called "Wurster" process. Wurster-type spray-coating equipment comprises a cylindrical outer vessel having a perforated floor through which a heated gas passes upwardly to heat and fluidize a batch of metal particles to be coated therein. A concentric, open-ended, inner cylinder is suspended above the center of the perforated floor of the outer vessel. A spray nozzle centered beneath the inner cylinder sprays a solution of the shell-forming polymer, dissolved completely in a solvent, upwardly into the inner cylinder (i.e., the coating zone) as the fluidized metal particles pass upwardly through the spray in the inner cylinder. The particles circulate upwardly through the center of the inner cylinder and downwardly between the inner and outer cylinders. The gas (e.g., air) that fluidizes the metal particles also serves to vaporize the solvent causing the dissolved shell-forming polymer to deposit as a smooth continuous film onto each particle's surface. After repeated passes through the coating zone in the inner cylinder, a sufficient thickness of polymer accumulates over the entire surface of each particle as to completely encapsulate such particle. Other spray-coating processes/apparatus are described in Smith-Johnson 3,992,558; Lindlof et al. 3,117,027; Reynolds 3,354,863; Wurster 2,648,609 and Wurster 3,253,944.
It is known to mechanically admix certain insoluble particulate additives with both encapsulated and unencapsulated metal particles. Hence for example, it is known to admix lubricant particles with such metal particles--see, for example, U.S. Patent Rutz et al. 5,198,137. It is likewise known to coat the surfaces of magnetic rare earth metal alloy particles with an antioxidant to provide long term magnetic stability--see, for example, Shain et al. supra. Still further, it is known to admix alloyant particles with unencapsulated metal particles for alloying with the metal particles during sintering e.g., see Semel 4,834,800. For example, graphite particles have heretofore been admixed with iron particles to carburize the iron during sintering. Other common alloyant particles for sinter-alloying include nickel, copper, molybdenum, sulfur and tin which may be either dry mixed with the metal particles or wet mixed in the presence of a solution of a binder such that upon drying the alloyant particles are bonded to the metal particles and to each other. Such alloyants comprise a minority amount of the particulate mass (i.e., less than about 5%-6% by weight). Finally, it is known to admix inert fillers (e.g., talc, quartz, etc.)with encapsulated metal particles, e.g., see Ebling 3,725,521.